The answer would be B.) Theorem :)
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An isosceles triangle has 2 sides of the same length and one side that's not the same length.
So it says the shortest side, y, is 2.1m, so that means it's the side that is not the same length as the other two sides.
But the other two sides will be the same length. Let's make those 2 sides x.
The equation would be
x + x + 2.1 = 7.5
And with that you can find x.